1. Field of the Invention
The invention relates to an electrical cable, and in particular, to a spirally wound cable that enhances the cancellation of magnetic fields and better controls differential mode impedance and common mode impedance.
2. Background Information
Twisted pair cable is a form of wiring in which two signal conductor wires are wound together for the purpose of canceling out electromagnetic interference (EMI) from external sources, electromagnetic radiation from the cable, and/or crosstalk between signal conductor pairs. The typical twisted pair cable operates in a differential mode where the two wires carry equal and opposite signals, and in a common mode where the two wires carry equal signals in the same direction.
The twisted pair cable generates magnetic fields around each of the two conductor wires. So as to produce equal and opposite magnetic fields that cancel each other out, the two wires should be uniformly wound. If the two wires in the given twisted pair cable are not identically wound, generated magnetic fields will not be exactly equal and opposite and so will not exactly cancel. However, it is difficult to twist the two wires in the twisted pair cable uniformly or precisely throughout the cable.
When the two wires are driven differentially, i.e., in the differential mode, signals propagating along the wires will encounter differential mode impedance. The differential mode impedance is determined by the size of the wires, the spacing between the two wires, and type of dielectric used between them. However, the twisted pair cable includes individual loosely paired wires that are hard to wind uniformly. Due to the non-uniform winding of the two wires, the distance between the two wires may not be identical along the twisted pair cable. Accordingly, the twisted pair cable may have imprecise differential mode impedance per unit length of the cable.
The typical balanced twisted pair cable does not have a return wire. When the two balanced twisted pair wires are unbalanced and carry a common mode current, the wiring experiences common mode impedance to the earth, commonly known as displacement capacitance. Also, in the twisted pair cable, the two signal conductor wires are twisted about each other. Further, due to the non-uniform twisting of the two wires, some portions may be tightly twisted, while other portions loosely twisted, and the number of twists per unit distance may not be uniform throughout its total length, resulting in unequal cancellation of the magnetic fields.
Further, in the twisted pair cable, an insulative jacket surrounds each twisted signal conductor pair. For example, a first insulative jacket may surround a first signal wire, while a second insulative jacket may surround a second signal wire. Therefore, between the first signal wire and the earth, the twisted pair cable may have the first insulative jacket on the first wire and the second insulative jacket on the second signal wire, with varying dielectric thickness. The non-uniform distance between the pairs, inconsistent twisting, and varying dielectric thickness make it difficult to control the common mode impedance per unit length of the cable.